The present invention relates to compositions for fibrous monolith ceramic and cermet composites and methods and apparatus for preparing the same.
Monolithic ceramic materials are known to exhibit certain desirable properties, including high strength and high stiffness at elevated temperatures, resistance to chemical and environmental attack, and low density. However, monolithic ceramics have one property that greatly limits their use in stressed environments, namely their low fracture toughness. While significant advances have been made to improve the fracture toughness of monolithic ceramics, mostly through the additions of whisker and particulate reinforcements or through careful control of the microstructural morphology, they still remain extremely damage intolerant. More specifically, they are susceptible to thermal shock and will fail catastrophically when placed in severe stress applications. Even a small processing flaw or crack that develops in a stressed ceramic cannot redistribute or shed its load on a local scale. Under high stress or even mild fatigue, the crack will propagate rapidly resulting in catastrophic failure of the part in which it resides. It is this inherently brittle characteristic which can be even more pronounced at elevated temperatures, that has not allowed monolithic ceramics to be utilized in any safety-critical designs.
Continuous fiber reinforced ceramic matrix composites (CFCCs) are improved composite materials that are better suited for use in high temperature and high stress applications. The use of fiber reinforcements in the processing of ceramic and metal matrix composites is known in the prior art, and has essentially provided the fracture toughness necessary for ceramic materials to be developed for high stress, high temperature applications. See J. J. Brennan and K. M. Prewo, xe2x80x9cHigh Strength Silicon Carbide Fiber Reinforced Glass-Matrix Composites,xe2x80x9d J. Mater. Sci., 15 463-68 (1980); J. J. Brennan and K. M. Prewo, xe2x80x9cSilicon Carbide Fiber Reinforced Glass-Ceramic Matrix Composites Exhibiting High Strength Toughness,xe2x80x9d J. Mater. Sci., 17 2371-83 (1982); P. Lamicq, et al., xe2x80x9cSiC/SiC Composite Ceramics,xe2x80x9d Am. Ceram. Soc. Bull., 65 [2] 336-38 (1986); T. I. Mah, et al., xe2x80x9cRecent Developments in Fiber-Reinforced High Temperature Ceramic Composites,xe2x80x9d Am. Ceram. Soc. Bull., 66 [2] 304-08 (1987).; K. M. Prewo, xe2x80x9cFiber-Reinforced Ceramics: New Opportunities for Composite Materials,xe2x80x9d Am. Ceram. Soc. Bull., 68 [2] 395-400 (1989); H. Kodama, et al., xe2x80x9cSilicon Carbide Monofilament-Reinforced Silicon Nitride or Silicon Carbide Matrix Composites,xe2x80x9d J. Am. Ceram. Soc., 72 [4] 551-58 (1989); and J. R. Strife, et al., xe2x80x9cStatus of Continuous Fiber-Reinforced Ceramic Matrix Composite Processing Technology,xe2x80x9d Ceram. Eng. Sci. Proc., 11 [7-8] 871-919 (1990).
Fibrous monoliths (FMs) are a unique class of structural ceramics. They have mechanical properties similar to CFCCs, including very high fracture energies, damage tolerance, and graceful failures but can be produced at a significantly lower cost. FM composites exhibit fracture behavior similar to continuous fiber reinforced ceramic composites (CFCC), such as C/C and SiC/SiC composites, including the ability to fail in a non-catastrophic manner. Unlike CFCC""s, Fibrous Monoliths are manufactured by polymer processing techniques using inexpensive raw materials. See D. Kovar, et al., xe2x80x9cFibrous Monolithic Ceramicsxe2x80x9d J. Am. Ceram. Soc., 80, [10] 2471-2487 (1997); G. E. Hilmas, et al., xe2x80x9cFibrous Monoliths: Non-Brittle Fracture from Powder Processed Ceramics,xe2x80x9d Mat. Sci. and Eng. A., 195, 263-268 (1995); G. E. Hilmas, et al., xe2x80x9cSiC and Si3N4 Fibrous Monoliths: Non-Brittle Fracture From Powder Processed Ceramics Produced by Coextrusion,xe2x80x9d Vol. 51 Ceramic Processing Science and Technology, pp. 609-14 (1993).
Methods of processing and fabricating fibrous monolith composites are known. U.S. Pat. No. 4,772,524 discloses a process that involves coating a fugitive cotton thread with a suspension of a first component that comprises the core material and then passing the coated thread through a second suspension of second component which comprises an interface material to form a bi-component coating on the cotton fiber. These bi-component fibers are then arranged to form a green fibrous monolith body. The fibrous monolith is then sintered. U.S. Pat. No. 5,645,781 describes a process for the preparation of fibrous monoliths from green monofilament ceramic fibers that have a controlled texture. The process involves (1) blending a thermoplastic polymer with at least 40 volume % of a ceramic powder and forming a substantially cylindrical core (2) blending a thermoplastic polymer with at least 40 volume % of a ceramic powder that differs compositionally from the powder contained in the core and applying it as a layer (commonly referred to as a xe2x80x9cshellxe2x80x9d or xe2x80x9ccladdingxe2x80x9d) onto the core to form a substantially cylindrical feed rod (3) extruding the feed rod to form a green ceramic monofilament with a smaller diameter then that of the feed rod (4) arranging the green monofilament into a green fibrous monolith body. The green fibrous monolith body is sintered to provide a fibrous monolith.
Fibrous monolith composites prior to the present invention have traditionally been fabricated discontinuously using methods and apparatuses to fabricate a feed rod of one composition and shell of a second composition that are co-extruded using a high pressure ram extruder. Although these methods and apparatuses produce a length of green fibrous monolith filament, the length of the co-extruded filament is limited by the size and volume of the feed rod and shell. It is therefore a discrete, batch process.
Thus, there exists a need for more efficient methods and apparatuses for applying a green material coating to a green matrix filament to produce a green fibrous monolith filament to any desired length that can be formed and finished to provide a composite structure exhibiting improved mechanical properties. There exists a further need for methods and apparatuses that are versatile enough to allow almost limitless combinations of matrix and interface coatings in the co-extrusion of a fibrous monolith composite. There exists a further need for methods and apparatuses that are versatile enough to allow almost limitless combinations of matrix and continuous fibers of any type to produce a continuous fiber reinforced composite.
The present invention overcomes the problems encountered with conventional compositions and methods by providing an efficient, cost-effective process for preparing multi-component filaments. More specifically, the invention provides compositions and methods for continuous co-extrusion and fabrication of fibrous monolith composites. The methods and apparatuses of the present invention are used to fabricate composite materials via economical, versatile, and controlled continuous composite co-extrusion processes. In a preferred embodiment of the present invention, a green ceramic monofilament fiber is introduced during melt extrusion of a thermoplastic loaded ceramic and/or metal. The result of this co-extrusion process is a coextruded xe2x80x9cgreenxe2x80x9d filament containing an in-situ thermoplastic loaded ceramic and/or metal core of one composition surrounded by a uniform coating thermoplastic loaded ceramic and/or metal interface of a second composition than differs from that of the core.
More specifically, the present invention relates to continuous processes for the fabrication of a fibrous monolith composite, i.e., a composite that is comprised of a core of a material, such as a ceramic or metallic material, in an architecture that is substantially a filament that is encased in an interface boundary composed of a ceramic and/or metal material of a second composition. A plurality of these filaments are put together to form a fibrous monolith composite body. A preferred method of the present invention comprises: (a) forming first a material-laden composition comprising a thermoplastic polymer and at least about 40 volume % of a ceramic or metallic particulate; (b) melt extrusion of the first material laden composition in the form of a filament (c) forming a second material-laden composition comprising a thermoplastic polymer and at least about 40 volume % of a ceramic or metallic particulate of a different composition; (d) melt extrusion of the second material-laden composition as the filament of the first material laden composition is simultaneously pulled through an extrusion die to form a continuous green fibrous monolith filament consisting of a xe2x80x9cgreenxe2x80x9d material of the first material-laden composition that is uniformly coated by second material-laden composition (e) arranging the continuous bicomponent filament into a desired architecture to provide a green fibrous monolith composite. The green composite may be subsequently fired, i.e., heated, to provide a fully consolidated and densified fibrous monolith composite with non-brittle failure characteristics.
The present invention also provides a process for the fabrication of a continuous fiber reinforced composite, i.e. a composite that is comprised of a matrix of material, such as a ceramic or metallic material, having fibers of a ceramic material dispersed within the matrix as a reinforcement. The process for forming a CFCC is (a) forming first a material-laden composition comprising a thermoplastic polymer and at least about 40 volume % of a ceramic or metallic particulate; (b) melt extrusion of the a material-laden composition while simultaneously pulling a ceramic fiber or fiber tow through an extrusion die; (c) arranging the continuous bicomponent filament into a desired architecture to provide a green fiber reinforced composite. The green composite may be subsequently fired, i.e., heated, to provide a fiber reinforced composite with non-brittle failure characteristics.
The present invention further provides methods for the fabrication of continuous filaments used in preparing fiber reinforced composites wherein the architecture of the filaments can be readily controlled.
Yet another aspect of the present invention is the ability to form the green filaments into a shaped green-body having a desired architecture. As one example, the extruded filament is molded by pressing into an appropriate mold at temperature of at least about 80xc2x0 C. The molding operation joins the green filaments together, creating a solid, shaped green body. Any shape that can be compression molded or otherwise formed by plastic deformation can be obtained with the extruded green filament. The green body so molded has the desired texture created by the arrangement of the extruded filaments. For example, a uniaxially aligned fibrous monolith composite can be obtained by a uniaxial lay-up of the extruded filaments prior to molding, or a woven architecture can be obtained by molding a shape from previously woven extruded filaments. The extruded filament product permits a wide variety of composite architectures to be fabricated in a molded green body.
In a preferred method of the present invention, a co-axial filament is produced with a green fiber core of a first material laden composition surrounded by a green coating of a second material laden composition.
The processing techniques of the invention readily allow for control of the fiber core and coating volume fraction and changes to the matrix composition. This technology is readily applicable to other matrix/fiber combinations and significantly enhances manufacturing capabilities for low cost, high-performance and high temperature ceramic composites.